1. Field of the Invention
This invention generally relates to packet communications and, more particularly, to a system and method of coding a G.709 Digital Wrapper frame with inner and outer coding.
2. Description of the Related Art
Digitally wrapped, or multidimensional frame structure communications generally describe information that is sent as a packet, with overhead to control the communication process. The packet can also include forward error correction (FEC) to recover the payload if the communication is degraded. One example of such communication is the synchronous optical network (SONET). Another example is the Digital Wrapper (DW) format often used in transporting SONET communications.
There are many framed communication protocols in use, depending on the service provider and the equipment being used. These differences in protocols can be arbitrary or supported by an underlying function. Frame synchronization and overhead placement are sometimes standardized by governing organizations such as the ITU-T. At the time of this writing, the ITU-T standard for the Digital Wrapper format is G.709.
Conventionally, the interface node must include two sets of equipment. A communication in a first protocol is received at the first set of equipment (processor). The message is unwrapped and the payload recovered. Synchronization protocols must be established between the equipment set and a second set of equipment (processor). The payload can then be received at the second equipment set and repackaged for transmission in a different protocol.
FIGS. 1a and 1b are diagrams illustrating the interleaving process and the formation of an interleaved frame (prior art).
FIGS. 2A and 2B are diagrams illustrating the G.709 optical data unit (ODU) frame structure, and the ODU, optical channel payload unit (OPU), optical channel transport unit (OTU) overhead, and parity section (prior art). More specifically, a superframe is shown that is composed of 4 rows (frames). In a G.709 compliant system, it is normal to provide read access to all 64 of the G.709 overhead bytes by dropping them to the user interface during each frame. Alternately stated, 16 overhead bytes are dropped from each row and 64 overhead bytes are dropped from each frame.
In G.709, the received BIP byte is calculated after errors have been corrected with the FEC. In order for the BIP in any individual frame to reflect any errors at all, the error density in one or more sub-rows of the frame must be higher than 8 bytes out of 255. This is because the standard FEC algorithm used in G.709 networks is the Reed-Solomon (255,239) algorithm which has the ability to correct all errors in a 255 byte codeword, if there are no more than 8 bytes out of each 255 bytes received in error. Thus, the Reed-Solomon (255,239) algorithm used in G.709 systems will correct up to 8 byte errors in each 255-byte block. If more than 8 bytes in each 255-byte block are in error, the Reed-Solomon (255,239) algorithm is no longer able to correct the errors.
It would be advantageous if a greater number of errored bytes could be corrected in a communicated DW G.709 superframe.
It would be advantageous if a DW G.709 were able to recover more data in the presence of long bursts of noise.